Science / Medicine : Crustacean Carries Its Own Bodyguard to Ward Off Predators

A shrimplike crustacean that lives in the frigid Antarctic waters under the polar icecap has no defense of its own against fish that would prey on it, but it has nonetheless developed a unique form of protection: a bodyguard.

It captures another organism, nearly as big as itself, that is distasteful to predators and carries it around on its back to ward off fish, according to biologists James B. McClintock of the University of Alabama at Birmingham and John Janssen of Loyola University in Chicago.

Fish that normally prey on the quarter-inch-long crustacean have learned to avoid it when it is carrying its distasteful companion, the researchers said in last week’s edition of the British journal Nature. Those who do try to eat the pair, the researchers added, shake their heads from side to side and spit them out violently with no harm to either the crustacean or its defender.

Carrying around its large, vile-tasting companion places a huge toll on the crustacean, called an amphipod: its swimming speed is reduced by 40%, making it that much more difficult for it to capture food.

But apparently that energy deficit is more than offset by the amphipod’s ability to remain alive.

Such chemical defenses against predators are not uncommon in the ocean. Many fish are colonized by smaller organisms whose taste or odor serves as a deterrent to predation. But those cases are invariably symbiotic in nature, offering benefits to both the host and the colonizer.

The newly observed defense of the amphipod is apparently unique, McClintock said in a telephone interview, in that the foul-tasting companion, called a sea butterfly, is an unwilling ally that receives no apparent benefit from its conscription.

The amphipod captures the sea butterfly with four of its 14 armlike appendages, embedding the ends in a pincer motion and hoisting it onto its own back “like a backpack,” McClintock said.

When it is captured, the sea butterfly retracts into itself and stops feeding. It remains that way for as long as it is held by the amphipod. Although evidence indicates that a small amount of dissolved food may pass through the organism’s body wall, “that would only supply a small amount of nutrition, nothing like a normal feeding,” McClintock said.

He speculated that the amphipods release the sea butterflies before they are starved to death, however. “We’ve never seen an amphipod carrying around a dead (sea butterfly),” he said.

McClintock and Janssen observed this behavior in the wild in the waters of McMurdo Sound in Antarctica during a National Science Foundation-sponsored visit in the austral summer. They confirmed it by capturing all the species involved and studying them in aquariums.

They observed that the predator fish quickly ate all the amphipods they encountered that were not carrying the sea butterflies, but that they avoided those that were carrying them.